TWI405334B - Field effect transistor - Google Patents

Abstract

A field-effect transistor includes at least a channel layer, a gate insulating layer, a source electrode, a drain electrode, and a gate electrode, which are formed on a substrate. The channel layer is made of an amorphous oxide material that contains at least In and B, and the amorphous oxide material has an element ratio B/(In+B) of 0.05 or higher and 0.29 or lower.

Description

Field effect transistor

The present invention relates to a field effect transistor using an amorphous oxide.

Field effect transistors (FETs) include a gate electrode, a source electrode, and a drain electrode, and are electronic active devices that control the flow of current into a channel layer by a voltage applied to the gate electrode. The current between the source electrode and the drain electrode is controlled. FETs are specifically referred to as thin film transistors (TFTs) which serve as a channel layer using a thin film deposited on an insulating substrate such as a ceramic, glass, or plastic substrate.

The aforementioned TFTs are formed by using a thin film technique, and thus the TFTs have an advantage of being easily formed on a substrate having a relatively large area, and thus are widely used as a flat plate for use in, for example, a liquid crystal display. The drive of the display. In particular, an active liquid crystal display (ALCD) turns on/off each image pixel by using TFTs formed on a glass substrate. Furthermore, for future high performance organic LED displays (OLEDs), which effectively control the current of each pixel by TFTs. Further, a liquid crystal display having higher performance is realized in which peripheral circuits having a function of driving and controlling the entire image are formed on a substrate by using TFTs in the vicinity of an image area.

Most of the popular TFTs currently use polycrystalline germanium films or amorphous germanium films as channel layer materials. The amorphous 矽 TFTs for pixel driving and the high performance polysilicon TFTs for the entire image driving/control have been practically used.

The disadvantages of previously developed TFTs, including amorphous germanium TFTs and polycrystalline germanium TFTs, require a high temperature process in the fabrication of those devices which makes it difficult to form the TFTs on plastic sheets, films, or other similar substrates.

Meanwhile, development of an elastic display has become active in recent years, and one of the TFTs formed on a resin substrate such as a polymer plate member or a film has a function of a driving circuit of an LCD or an OLED. This attracts attention to an organic semiconductor film which can be deposited at a low temperature and which is electrically conductive as a material which can be deposited on a plastic film or the like.

An example of a pentacene-based organic semiconductor film, and its research and development has been advanced. It has been reported that the carrier mobility of the pentacene is about 0.5 cm ² /Vs, which is equivalent to the carrier mobility of amorphous 矽 TFTs.

However, pentacene and other organic semiconductors have problems of low thermal stability (<150 degrees Celsius) and are not successful in producing a device that can be used in practice.

Another material that has recently attracted attention as applied to the channel layer of the TFT is an oxide material. For example, TFTs used as a channel layer of a transparent conductive oxide polysilicon film having ZnO as a main component are actively developed. The film can be deposited at relatively low temperatures and formed on a plastic sheet, film, or other similar resin substrate. However, in general, a compound having ZnO as a main component cannot form a stable amorphous phase at room temperature, and instead forms a polycrystalline germanium phase, which causes electron scattering at the polycrystalline grain boundary and makes it difficult to increase the Electronic mobility. In addition, the electrical properties of the polycrystalline compound are greatly affected by the shape and interconnection of the polycrystalline grains, which can be determined by the manufacturing process of a thin film deposition condition, and thus in some cases, the resulting TET device Has a changing feature.

Regarding this issue, a thin film transistor using an amorphous oxide based on indium-gallium-zinc-oxygen has been reported in K. Nomura et al., Nature 432, pp. 488-492 (2004-11). in. The transistor can be formed on a plastic or glass substrate at room temperature. The transistor also features a normally closed transistor at a field effect mobility of about 6 to 9. Another advantageous feature is that the transistor is transparent to visible light. In particular, the foregoing document describes a technique using an amorphous oxide having a composition ratio of indium:gallium:zinc = 1.1:1.1:0.9 (atomic ratio) for the channel layer of the TFT.

Although amorphous metal oxides using three metal elements of indium, gallium, and zinc are employed in the foregoing documents, it is preferred to use a smaller number of metal elements from the viewpoint of composition control and ease of material adjustment.

On the other hand, when deposited by sputtering or the like, a polycrystalline film is formed substantially using a type of metal element such as ZnO and an oxide of In ₂ O ₃ , and accordingly, a TFT device is highly likely to be formed. Fluctuations in the characteristics, as described above.

The results of studies on indium-zinc-oxygen-based amorphous oxides are known from the application of Physical Communication 89,062103 (2006) as an example of the use of a two-type metal element. However, it is known that when the oxide is stored in the atmosphere, the resistivity of the amorphous-based oxide based on indium-zinc-oxygen can be varied, and thus the environment is desired to be improved.

In addition, the results of studies on amorphous oxides based on indium-zinc-oxygen have been reported in Solid State Electronics, 50 (2006), pages 500-503. In this report, a heat treatment of 500 degrees Celsius is performed, and thus it is desirable to manufacture a device at a lower temperature. This is because if the device can be fabricated at a low temperature, an inexpensive glass substrate or resin substrate can be used.

Therefore, in the field of the thin film transistor, a non-crystalline oxide which includes a small number of metal elements and has excellent stability is desired.

The present invention has been accomplished on the basis of the above-discussed problems, and an object of the present invention is to provide a field effect transistor which uses an amorphous oxide which includes a small number of elements and exhibits a large switch ratio. Furthermore, another object of the present invention is to provide a potent transistor having an excellent environmental stability for storage in the atmosphere.

The field effect transistor according to the present invention comprises at least one channel layer, a gate insulating layer, a source electrode, a drain electrode, and a gate electrode, which are formed on a substrate, wherein the channel layer is An amorphous oxide material comprising at least indium and boron, and wherein the amorphous oxide material has an elemental ratio B/(In+B) of 0.05 or higher and 0.29 or lower.

Furthermore, the field effect transistor according to the present invention comprises at least one channel layer, a gate insulating layer, a source electrode, a drain electrode, and a gate electrode, which are formed on a substrate, wherein the channel The layer is made of an amorphous oxide material comprising at least indium, zinc and boron, and wherein the amorphous oxide material has an element ratio B/(In+Zn+B) of 0.05 or higher and 0.29 or more low.

Furthermore, the display according to the present invention comprises: the field effect transistor; and at least one pixel device having an electrode connected to one of a source electrode and a drain electrode of the field effect transistor.

According to the present invention, the channel layer is made of a novel material such as an amorphous oxide containing indium and boron, and thus a thin film transistor exhibiting good characteristics can be realized. In particular, the present invention achieves such an effect that the transistor characteristics including the switching ratio and the S value are excellent, and the environmental stability is good.

The exemplary embodiments used to embody the invention are described.

The inventors of the present invention have earnestly studied an oxide material comprising a two-type metal element, such as an oxide containing indium (In) and boron (B), as a material of a channel layer of a thin film transistor. Figure 2 illustrates the temporal variation in the resistivity of various oxide films formed by sputtering. In FIG. 2, between indium and another metal element M, each film used has an element ratio M/(In+M) of about 0.2. As is apparent from FIG. 2, a temporary change in the resistivity of each of the oxide containing indium and zinc (In-Zn-O) and the oxide containing indium and tin (In-Sn-O) is big. In contrast, it becomes apparent that the temporary change in the resistivity of each of the oxide containing indium and boron (In-BO) and the oxide containing indium and gallium (In-Ga-O) hardly occur. The oxide (In-B-O) has excellent environmental stability in electrical properties and is thus a desired semiconductor material.

Second, the above materials are used in the channels to make prototypes of thin film transistors. In the case of each of oxide (In-Zn-O) and oxide (In-Sn-O), it is difficult to realize a transistor having a switching ratio equal to or greater than five orders of magnitude. In contrast, in the case of the oxide (In-BO), as is apparent from the transfer characteristic (Id-Vg curve) illustrated in FIG. 1, it is possible to realize that the switching ratio is equal to or greater than ten orders of magnitude. The transistor.

3A and 3B are diagrams in which when the oxide of In-M-O (metal element M is a group III element of the periodic table) is used for the channel, the crystal characteristics are compared with each other each time. As illustrated in FIG. 3A, a thin film transistor (TFT) including a channel containing In-BO exhibits a comparison with a TFT including a channel containing In-Ga-O and a TFT including a channel containing In-Al-O. Big switch ratio. As illustrated in FIG. 3B, the TFT including the channel containing In-BO exhibits a smaller S value than the TFT including the channel containing the In-Ga-O and the TFT including the channel containing the In-Al-O. . Therefore, the TFT using the In-B-O for this channel exhibits the desired transistor characteristics.

Next, a specific embodiment of the field effect transistor according to the present invention will be described in detail.

First, the structure of the field effect transistor according to this embodiment is described.

8A, 8B, and 8C are cross-sectional views illustrating an example of the structure of a field effect transistor according to this embodiment. In FIGS. 8A, 8B, and 8C, the substrate, the channel layer, the gate insulating layer, the source electrode, the drain electrode, and the gate electrode are denoted by reference numerals 10, 11, 12, 13, 14, and 15, respectively.

The field effect transistor system three-terminal device according to this embodiment includes the gate electrode 15, the source electrode 13, and the drain electrode 14. The field effect transistor has a function of applying a voltage Vg to the gate electrode, controlling a current Id flowing through the channel layer, and switching a current Id between the source electrode and the drain electrode.

FIG. 8A illustrates an example of a top gate structure in which the gate insulating layer 12 and the gate electrode 15 are continuously formed on the semiconductor channel layer 11. Furthermore, FIG. 8B illustrates an example of a bottom gate structure in which the gate insulating layer 12 and the semiconductor channel layer 11 are continuously formed on the gate electrode 15. Again, Figure 8C illustrates another example of the bottom gate transistor. In FIG. 8C, the substrate (n ⁺ germanium substrate, which also serves as the gate electrode), the insulating layer (SiO ₂ ), the channel layer (oxide), the source electrode, and the drain electrode are denoted as reference numerals 21, 22, respectively. , 25, 23 and 24.

The structure of the field effect transistor is not limited to the structure of the present invention as discussed above, and any of the top/bottom gate structures or staggered/inverted staggered structures may be used.

Next, the components constituting the field effect transistor of this embodiment will be described in more detail.

(channel layer)

As described above, the field effect transistor of this embodiment is characterized in that an amorphous oxide containing at least indium and boron is used for the channel layer. A non-crystalline oxide (In-B-O) containing indium and boron and a non-crystalline oxide (In-Zn-B-O) containing indium, boron and zinc are particularly desirable materials. Non-crystalline oxides containing indium, tin, and boron are also available.

In this embodiment, the ratio of boron to all of the metal elements contained in the amorphous oxide is equal to or greater than 10 atom% and equal to or less than 40 atom%. For all of these elements, the amorphous oxide made from In-B-O comprises a maximum amount of oxygen, a second maximum amount of indium, and a third maximum amount of boron. For all of these elements, the amorphous oxide made of In-Zn-BO comprises a maximum amount of oxygen, a second maximum amount of zinc (or indium), a third maximum amount of indium (or zinc), and The fourth largest amount of boron.

Using the amorphous oxide comprising indium and boron for the channel layer, the inventors of the present invention identified the following during the careful study of the thin film transistor. That is, when an oxide semiconductor system having a specific element ratio B/(In+B) is applied to the channel, a transistor exhibiting a desired feature can be realized.

In the use of In-B-O for the channel of the thin film transistor, there is a preferred ratio of indium to boron. The preferred element ratio B/(In+B) is 0.05 (5 atom%) or higher because, at this element ratio, the amorphous film can be obtained by sputter deposition, so that the substrate temperature is maintained at Room temperature. This is because, as described above, the polycrystalline phase causes fluctuations in the characteristics of the TFT device, in which the shape and interconnection of the polycrystalline grains largely vary depending on the film deposition method.

Fig. 5 is a graph showing an example of the dependence of the field effect mobility ratio on the element ratio B/(In+B) when a field effect transistor using an In-BO film is fabricated. As is apparent from Fig. 5, when the content of boron is decreased, the field effect mobility becomes large. The required value of the field effect mobility depends on the use of the field effect transistor. For example, an electrophoretic display can be driven by a TFT having a field effect mobility of about 0.05 cm ² /Vs. The liquid crystal display can be driven by a TFT having a field effect mobility of about 0.1 cm ² /Vs. In an organic EL display, a desired field effect mobility is equal to or greater than 1 cm ² /Vs. From this point of view, the element ratio B/(In+B) between indium and boron is desirably equal to or less than 0.29, more desirably equal to or less than 0.22, and most desirably equal to or less than 0.2.

Figure 6 is a graph showing the results obtained by studying the compositional dependence of the threshold voltage of a field effect transistor based on In-B-O. When the threshold voltage Vth of the field effect transistor based on In-B-O is equal to or greater than 0 volt, a circuit is easily established. As illustrated in FIG. 6, the element ratio B/(In+B) is desirably equal to or greater than 0.1 because the threshold voltage Vth is positive in this case.

Figure 7 is a graph illustrating the results obtained by studying the crystal characteristics of a field effect transistor based on In-B-O. As illustrated in FIG. 7, when the element ratio B/(In+B) is set to a value equal to or greater than 0.12 and equal to or less than 0.2, a transistor having a smaller S value can be realized.

It concludes from the above that in the case of using In-BO for the channel layer of the field effect transistor, the element ratio B/(In+B) between indium and boron is desired to be 0.05 or higher and 0.29 or more. Low, more desirable is 0.1 or higher and 0.22 or lower, and most desirably 0.12 or higher and 0.2 or lower.

The thickness of the oxide (channel layer) in this embodiment is desirably equal to or greater than 10 nm and equal to or less than 200 nm, more desirably equal to or greater than 20 nm and equal to or less than 70 nm. And the most desirable ground is equal to or greater than 25 nm and equal to or less than 40 nm.

Further, in order to obtain excellent TFT characteristics, the conductivity of the amorphous oxide film used as the channel layer is desirably set to 0.000001 S/cm or more and 10 S/cm or less. When the conductivity is greater than 10 S/cm, it is difficult to obtain a normally closed transistor and increase the switching ratio. In an extreme case, the application of the gate voltage cannot open/close the current between the source and the drain electrode, and the TFT does not function as a transistor. On the other hand, when the conductivity is smaller than 0.000001 S/cm, the oxide film is caused to be an insulator, which is difficult to increase the on current. In an extreme case, the application of the gate voltage cannot open/close the current between the source and the drain electrode, and the TFT does not function as a transistor.

Although the material composition of the channel layer also obtains a factor of the desired range of the aforementioned conductivity, the amorphous oxide film desirably has an electron carrier concentration of about 10 ¹⁴ to 10 ¹⁸ /cm ³ .

The conductivity of the oxide applied to the channel layer is controlled based on the composition ratio of the metal element, the oxygen partial pressure during film deposition, and the annealing conditions after the film formation. Figure 11 is a graph showing the dependence of the resistivity of the In-B-O semiconductor film on the element ratio B / (In + B). It becomes apparent that when the element ratio B/(In+B) is increased, the conductivity becomes smaller (the resistivity thereof becomes larger). The oxygen partial pressure during film deposition is controlled to mainly control the oxygen deficiency in the oxide semiconductor film, whereby the electron carrier concentration can be controlled. Figure 11 illustrates the conductivity of a film deposited by sputtering at different oxygen partial pressures. It becomes apparent that the conductivity becomes smaller as the oxygen partial pressure during film deposition increases.

As described above, the electrical conductivity is controlled to some extent based on the film deposition conditions. However, there is a limit to controlling the conductivity, depending on the element ratio B/(In+B). For example, when the element ratio B/(In+B) is equal to or greater than 0.4, the film becomes substantially an insulator irrespective of the oxygen partial pressure during film deposition. In one case, the element ratio B/(In+B) is equal to or less than 0.1, and even when the oxygen partial pressure during film deposition increases, it is difficult to form a semiconductor film having the conductivity to one. A value equal to or less than 1 S/cm.

In this embodiment, elements inevitably contained or elements having a content adversely affecting the characteristics other than indium, boron, and oxygen are allowed to be regarded as elements contained in the amorphous oxide.

(gate insulation layer)

The material of the gate insulating layer 12 is not particularly limited as long as the material has excellent insulating properties. However, what it wants is to use a film whose main component is ruthenium because of its excellent crystal characteristics. Although it is not fully understood why these features become excellent, the reason is considered to be that an excellent interface can be formed between the channel layer containing boron and the gate insulating layer whose main component is germanium.

In particular, desirable examples of materials whose main component is niobium include niobium oxide SiO _x , niobium nitride SiN _x , and niobium oxynitride SiO _x N _y . Further, Si-BO, Si-Hf-O, Si-Al-O or Si-YO can be used as a composite oxide whose main component is ruthenium.

By using a film having excellent insulating properties, as described above, the leakage current can be reduced to between about 10 and ¹¹ between the source and gate electrodes and between the drain and gate electrodes. ampere.

The thickness of the gate insulating layer is, for example, about 50 to 300 nm.

(electrode)

Each of the source electrode 13, the drain electrode 14, and the gate electrode 15 is not particularly limited as long as excellent electrical conductivity is obtained, and electrical connection to the channel layer is possible.

For example, a transparent conductive film such as In ₂ O ₃ :Sn or ZnO, or a metal electrode such as gold, nickel, tungsten, molybdenum, silver, or platinum can be used. Any layered structure including a gold-titanium layered structure may also be employed.

(substrate)

As the substrate 10, a glass substrate, a plastic substrate, a plastic film, or the like can be used.

The aforementioned channel layer and the gate insulating layer are transparent with respect to visible light, and thus it is possible to obtain a transparent field effect transistor by using a transparent material as each of the foregoing electrodes and substrates.

As a method of depositing the foregoing oxide thin films, a vapor phase process such as a sputtering method (SP method), a pulsed laser deposition method (PLD method), and an electron beam deposition method are provided. It should be noted that in these vapor phase processes, the sputtering method is suitable from the viewpoint of productivity. However, the film deposition method is not limited to those methods. Furthermore, the temperature of the substrate during film deposition can be maintained in a state where the substrate is not intentionally heated, in other words, at room temperature. Accordingly, the method can be performed during a low temperature process, and thus the field effect transistor can be formed on the substrate, such as a plastic sheet or foil.

Hereinafter, with reference to Figures 9A and 9B, the features of the field effect transistor of this embodiment are described.

FIG. 9A illustrates an example of Id-Vd characteristics obtained at various gate voltages Vg, and FIG. 9B illustrates an example of Id-Vg characteristics (transfer characteristics) when Vd=6V. The difference in the characteristics of the transistor can be expressed as the difference in field effect mobility μ, threshold voltage (Vth), switching ratio, and S value.

The field effect mobility can be obtained from features of a linear region or a saturated region. For example, it is possible to use a method of creating a graph representing Id ^1/2 -Vg from the results of the transition features in order to obtain the field effect mobility from the slope of the graph. In the context of the present invention, evaluation is performed by this method unless otherwise indicated.

While some methods of obtaining the threshold are provided, the threshold voltage Vth can be obtained from an x-thick such as a curve representing Id ^{1/2 -} Vg.

The switch ratio is obtained from the ratio of the maximum Id value of the transfer characteristics to the minimum value of Id.

The S value can be obtained by reciprocal of the inclination of the graph of Log(Id)-Vg, which is established from the results of the transfer characteristics.

The difference in the characteristics of the transistors is not limited to the above, but can also be represented by various parameters.

A semiconductor device like an active matrix substrate is provided with a field effect transistor according to this embodiment, as described above. A transparent semiconductor device is provided with a transparent substrate and transparent amorphous oxide TFTs. Therefore, when the transparent active matrix substrate is applied to a display, the aperture ratio of the display can be increased. In particular, when the active matrix substrate is used for an organic EL display, a structure in which light is radiated from the side (bottom of the substrate) can be used. An active matrix substrate provided with a field effect transistor in accordance with this embodiment can be used in a variety of applications such as ID tags or IC tags.

Hereinafter, a display is specifically described as an example of an active matrix substrate provided with a field effect transistor according to this embodiment.

A drain electrode as an output terminal of a field effect transistor according to this embodiment is connected to an electrode of a pixel device, such as an organic or inorganic electroluminescence (EL) device or a liquid crystal device, whereby the display can be production. Hereinafter, a specific structural example of the display is described with reference to a cross-sectional view of the display.

For example, as illustrated in Figure 12, a field effect transistor system is formed on a substrate 111. The field effect transistor includes a channel layer 112, a source electrode 113, a drain electrode 114, a gate insulating layer 115, and a gate electrode 116. The drain electrode 114 is connected to an electrode 118 via an interlayer insulating layer 117. The electrode 118 is brought into contact with a light-emitting layer 119. The luminescent layer 119 is brought into contact with an electrode 120. According to the structure as described above, the current injected into the light-emitting layer 119 can be controlled based on the current value flowing from the source electrode 113 to the drain electrode through the channel formed in the channel layer 112. 114. Therefore, the current can be controlled based on the voltage of the gate electrode 116 of the field effect transistor. The electrode 118, the light-emitting layer 119, and the electrode 120 function as inorganic or organic electroluminescent devices.

As illustrated in FIG. 13, a structure can be employed in which the drain electrode 114 is extended to also function as the electrode 118 and is used as the electrode 118 for applying a voltage to the high resistance film 121. And 122 liquid crystal cells or electrophoretic particle cells 123 sandwiched. The liquid crystal cell or electrophoretic particle cell 123, the high resistance films 121 and 122, and the electrodes 118 and 120 constitute a pixel device. The voltage applied to the pixel device can be controlled based on the value of the current flowing from the source electrode 113 to the drain electrode 114 through the channel formed in the channel layer 112. Therefore, the current can be controlled based on the voltage of the gate electrode 116 of the TFT. When the display medium of the pixel device is a capsule in which a fluid and particles are sealed in an insulating coating, the high resistance films 121 and 122 are not required.

In each of the above two examples, the thin film transistor typically has a staggered structure (top gate type). However, the present invention is not necessarily limited to the above structure. For example, when the gate electrode as the output terminal of the thin film transistor and the pixel device are connected so as to be identical in phase, another structure such as a coplanar type can be employed.

In each of the above two examples, the pair of electrodes for driving the pixel device are provided parallel to the substrate. However, the present invention is not necessarily limited to the above structure. For example, when the drain electrode as the output terminal of the thin film transistor and the pixel device are connected so as to be completely identical in phase, any one or both of the electrodes may be provided perpendicular to the substrate.

In the case where the pair of electrodes for driving the pixel device are provided in parallel to the substrate, when the pixel device is an EL device or a reflective pixel device, such as a reflective liquid crystal device, An electrode system needs to be transparent with respect to a wavelength of light emitted or a wavelength of reflected light. When the pixel device is a transmissive pixel device, such as a transmissive liquid crystal device, both of the electrodes need to be transparent with respect to the transmitted light.

The thin film transistor according to this embodiment can include all of the transparent constituent elements, and thus a transparent display can be formed. The display can also be placed on a low thermal resistance substrate, such as a plastic substrate, which is lightweight, flexible, and transparent.

Next, referring to Fig. 14, a display in which a plurality of pixels are two-dimensionally arranged, each of which includes an EL element (in this embodiment, an organic EL element) and a field effect transistor.

Figure 14 illustrates a transistor 201 for driving the organic EL layer 204 and a transistor 202 for pixel selection. A capacitor 203 is provided to maintain the selected conditions and to store the charge between the common electrode line 207 and the source portion of the transistor 202, thereby maintaining the gate signal of the transistor 201. A pixel to be selected is determined by the scan electrode line 205 and a signal electrode line 206.

More specifically, an image signal is applied as a pulse signal from the driver circuit (not shown) to the gate electrode through the scan electrode line 205. At the same time, a pulse signal is applied to the transistor 202 via the signal electrode line 206 by another driver circuit (not shown) to select the pixel. At this time, the transistor 202 is turned on, whereby the charge is stored in the capacitor 203 between the signal electrode line 206 and the source portion of the transistor 202. Then, the gate voltage of the transistor 201 is maintained to a desired voltage to turn on the transistor 201. This state is maintained until the next signal is received. During this state, in which the transistor 201 is turned on, a voltage and a current are continuously supplied to the organic EL layer 204, thereby maintaining light emission.

Figure 14 illustrates this structural example in which each pixel includes a di-electrode and a capacitor. However, to improve this performance, for example, a larger number of transistors can be incorporated in each pixel.

(example)

The following description is an example of the invention. However, the present invention is not limited to the following examples.

Example 1

In this example, the top gate TFT device illustrated in FIG. 8A is formed to have a non-crystalline oxide based on In-B-O as a channel layer.

An amorphous oxide film based on In-B-O was first formed as a channel layer on a glass substrate (1737 manufactured by Corning Incorporated). In particular, the In-B-O-based amorphous oxide film is formed by a radio frequency sputtering method in a mixed atmosphere of argon gas and oxygen gas. At this time, a sputtering deposition apparatus as illustrated in FIG. 10 is used. In FIG. 10, the sample, the target, the vacuum pump, the vacuum gauge, and the substrate holder are denoted by reference numerals 31, 32, 33, 34, and 35, respectively. A gas flow controller 36 is provided for each gas introduction system. A pressure controller and a thin film deposition chamber are denoted by reference numerals 37 and 38, respectively. The vacuum pump 33 is an exhaust unit for evacuating the inside of the thin film deposition chamber 38. The substrate holder 35 is a unit for holding the substrate on which the oxide film is to be formed in the thin film deposition chamber. The target (solid material source) 32 is placed facing the substrate holder. The deposition apparatus is further provided with an energy source (RF power source) (not shown) for causing the material to evaporate from the target 32, and a unit for supplying gas to the interior of the film deposition chamber.

The deposition apparatus has a two gas introduction system, one system for argon and the other system for a mixed gas of argon and oxygen (Ar: O ₂ = 95: 5). A predetermined gas atmosphere can be obtained in the film deposition chamber by a gas flow controller 36 capable of individually controlling the respective gas flow rates of the apparatus, and a pressure controller 37 for controlling the exhaust ratio.

In this example, as a goal, the target of the 2 吋 size design of In ₂ O ₃ and B ₂ O ₃ was used to form an In-BO film by simultaneous sputtering. For this individual target, the input RF power is 70 watts and 35 watts. The atmosphere during deposition of the film was set such that the total pressure was 0.4 bar and the gas flow ratio was Ar:O ₂ = 100:1. The film deposition ratio and the substrate temperature were set to 12 nm/min and 25 degrees Celsius, respectively. After the film was deposited, the film was subjected to an annealing process at atmospheric pressure of 300 degrees Celsius for 60 minutes.

A cut-incident X-ray diffraction analysis was performed on the surface of the thus obtained film (film method, incident angle: 0.5 degree). No significant diffraction peaks were detected, indicating that the formed In-B-O based film was an amorphous film.

Spectral ellipsometry measurements were further performed on the pattern analysis to reveal that the film had a root mean square (Rrms) roughness of about 0.5 nm and a thickness of about 30 nm. An inductively coupled plasma emission spectrometry method (ICP method) is used to analyze the ratio of the metal components to reveal that the ratio of the metal composition of the film is In:B=85:15. In other words, it was found that the element ratio B/(In+B) was 0.15.

The conductivity, the electron carrier concentration, and the electron mobility were evaluated and estimated to be about 10 ^-3 S/cm, 2 x 10 ¹⁵ /cm ³ , and about 4 cm ² /Vs, respectively.

The drain electrode 14 and the source electrode 13 are formed and patterned by lithography and a photoresist peeling method. The material of the electrodes is a gold-titanium layered film. The thickness of the gold layer is 40 nm, and the thickness of the titanium layer is 5 nm.

The gate insulating layer 12 is formed and patterned by lithography and a photoresist peeling method. The gate insulating layer 12 is deposited by sputtering deposition to a 150 nm thick SiO ₂ film. The specific dielectric constant of the SiO ₂ film is about 3.7.

The gate electrode 15 is also formed by lithography and a photoresist peeling method. The length of the channel and the width of the channel are 50 microns and 200 microns, respectively. The material of the electrode is gold and its thickness is 30 nm. The TFT is fabricated in the above manner.

Second, the characteristics of the TFT are evaluated.

9A and 9B illustrate examples of current-voltage characteristics of TFTs measured at room temperature. Figure 9A illustrates the Id-Vd feature, whereas Figure 9B illustrates the Id-Vg feature. In FIG. 9A, the dependence of the source-drain current Id on the gate voltage Vd caused by the change in Vd is measured under application of a constant gate voltage Vg. As illustrated in Figure 9A, saturation (clamping) is observed at approximately Vd = 6 volts, which is a typical semiconductor transistor behavior. The TFT characteristics are such that the threshold is about 1.5 volts at Vd = 6 volts. At Vg = 10 volts, the flow Id = a current of approximately 1.0 x 10 ^-4 amps.

Moreover, the switching ratio of the transistor exceeds 10 ⁹ . In this saturated region, the field effect mobility calculated from the output characteristics is approximately 2.5 cm ² /Vs.

The TFTs produced in this example have excellent repeatability, and the fluctuations in the features in most of the devices produced are small.

As described above, when a novel amorphous oxide of In-B-O is applied to the channel layer, excellent transistor characteristics can be realized. In particular, there is an advantage in that the type of constituent elements is less than the conventional oxide based on In-Ga-Zn-O. Therefore, it can be expected to reduce manufacturing load and manufacturing cost.

Example 2

In this example, the In-B composition is checked for a thin film transistor having a channel layer containing indium and boron as main components.

This example uses a method for the combination of film deposition to check the material composition of the channel layer. That is, the inspection is performed by a method of forming a film of an oxide having various compositions on a single substrate by sputtering immediately. However, it does not require the technique of applying this combination. A desired ingredient (source of material) can be prepared to deposit a film for each component. Films having various compositions can also be formed by separately controlling the input power for most targets.

The In-BO film is formed by the use of a ternary cut incident sputtering apparatus. The target is positioned obliquely relative to the substrate, and the composition of the film on the surface of the substrate varies due to the difference in distance from the target. As a result, a film having a broad compositional layer on the substrate can be obtained. In forming the In-BO film, the target of In ₂ O ₃ and a target of B ₂ O ₃ are simultaneously subjected to sputtering. The input RF power is set to 35 watts and 70 watts, respectively. The atmosphere during the deposition of the film was set such that the total pressure was 0.35 bar and the gas flow ratio was Ar:O ₂ = 100:1. The substrate temperature was set to 25 degrees Celsius.

The physical properties of the thus formed film are evaluated by fluorescing X-ray analysis, spectral ellipsometry, X-ray diffraction, and four-point detection resistivity measurement. Further, a combined TFT library (TFT having various channel components on a substrate) is formed by classifying a film using an In-B-O composition. The TFT structure is a bottom gate and a top contact type.

In the region where the element ratio B/(In+B) is 0.05 or higher, it was confirmed by X-ray diffraction (XRD) measurement that the formed In-B-O film was amorphous. In the partial film region, in which the element ratio B/(In+B) is smaller than 0.05, the diffraction peak of the crystal is observed. From the results discussed above, it can be concluded that an amorphous film can be obtained by setting the element ratio B/(In+B) in the In-B-O film to 0.05 or higher.

The sheet resistance of the In-B-O composition graded film was measured by a four-point detection method, and the thickness of the film was measured by spectral ellipsometry to obtain the resistivity of the film. As a result, as explained in Fig. 11, it is confirmed that the resistivity changes with the element ratio B/(In+B), and the resistance is found in the indium-rich component (the element ratio B/( In+B) is low on the small component and high on the boron-rich component (the element ratio B/(In+B) is large).

Further, when the partial pressure of oxygen in the film deposition atmosphere has been changed, the resistivity of the graded film of the In-B-O component is obtained. It was found that as illustrated in Fig. 11, the electric resistance of the In-B-O film was increased due to an increase in oxygen partial pressure. This may be due to a decrease in oxygen deficiency and a decrease in the concentration of the electron carrier. It has also been found that the resistance is suitable for the composition of the active layer of the TFT to vary with respect to the oxygen partial pressure.

Figure 15 illustrates the measurement of the resistivity of the In-B-O film as a function of time. Throughout a broad range of compositions (the element ratio B/(In+B) is in the range of 0.05 to 0.5), no change in resistivity with time was observed in the film based on this In-B-O. Furthermore, Figure 2 illustrates the comparison of resistivity over time among other materials. The In-Zn-O film and the In-Sn-O film formed in the same manner as the In-B-O film exhibited a tendency to reduce the resistivity with time. Those results confirmed that the In-B-O film had a superior environmental stability.

Next, the characteristics and composition dependence of the field effect transistor (FET) having the In-BO film as the n-channel layer are examined. The transistor has a bottom gate structure as illustrated in Figure 8C. In particular, an In-BO composition graded film is formed on a tantalum substrate having a thermal oxide film, and then patterning and electrode formation are performed, thereby forming a device including active layers having different compositions from each other on a single substrate. . After the device is formed, an annealing process is performed in the atmosphere at 300 degrees Celsius. Many FETs are fabricated on 3-inch wafers and evaluated from a feature standpoint. The FETs have a bottom gate, top contact structure for which n ⁺ -Si is used for the gate electrode, SiO ₂ is used for the insulating layer, and Au/Ti is used for the source and drain electrodes. The channel layer width and the channel layer length are 150 microns and 10 microns, respectively. The source-drain voltage for the FET evaluation is 6 volts.

In the TFT feature evaluation, the electron mobility is obtained by the slope of Id ^1/2 with respect to the gate voltage (Vg) (Id: drain current), and the current switching ratio is determined by the maximum Id value. The ratio of the minimum Id value is obtained. When Id ^1/2 is plotted with respect to Vg, the intercept relative to the Vg axis is taken as the threshold voltage, and the minimum value of dVg/d(log Id) is taken as the S value (increasing the current by an order of magnitude) The required voltage value).

The change in the element ratio B/(In+B) is checked by evaluating the TFT characteristics by evaluating the TFT features at various locations on the substrate. It has been found that the TFT characteristics vary depending on the position on the substrate, that is, the element ratio B/(In+B). The Id-Vg characteristics of the various components are illustrated in FIG.

In the case of an indium-rich component (for example, (A) of Fig. 4), the turn-on current is large, and the turn-off current is relatively large. A device having the indium-rich composition can be used for applications requiring a large on-current. When a film having a smaller element ratio B/(In+B) than in the case of (A) of Fig. 4 is applied, the threshold is negative.

In contrast, in the case of a boron-rich component (for example, or (C) or (D) of Figure 4, the turn-on current is relatively small, but the threshold voltage is positive, and thus a "normal" Closed feature." A device with a boron-rich composition can be used for applications that require a small shutdown current.

The device in the case of (B) of Figure 4 presents the most superior features. The features of this device (Fig. 4(B)) are also separately illustrated in Fig. 1. As illustrated in FIG. 1, in the case of the device having an element ratio B/(In+B) of 0.16 ((B) of FIG. 4), a switching ratio equal to or greater than ten orders of magnitude can be obtained. The S value is 0.4 V/decade, the field effect mobility is 3 cm ² /Vs, and the threshold voltage is 4V.

Figure 5 illustrates the element ratio B/(In+B) dependence of the field effect mobility. It can be seen that this field effect mobility increases as the boron content decreases. When the element ratio B/(In+B) is 0.29 or lower, a field effect mobility of 0.05 cm ² /Vs or higher is obtained. Furthermore, when the element ratio B/(In+B) is 0.22 or less, a field effect mobility of 0.1 cm ² /Vs or higher is obtained, and when the element ratio B/(In+B) is 0.2. At or lower, a field effect mobility of 1 cm ² /Vs or higher is obtained.

Furthermore, FIG. 6 illustrates the element ratio B/(In+B) dependence of the threshold voltage. In general, when the threshold voltage Vth of a thin film transistor is 0 volt or higher, a circuit is easily established. As can be seen from Fig. 6, when the element ratio B/(In+B) is 0.1 or higher, Vth becomes positive.

Figure 7 illustrates the element ratio B/(In+B) dependence of the S value. As is apparent from Fig. 7, when the element ratio B/(In+B) is equal to or greater than 0.12 and equal to or less than 0.2, a small S value is obtained, which is desirable.

In FIGS. 3A and 3B, the characteristics of the transistor to which In-Ga-O is applied to the channel layer are compared with the characteristics of the transistor in which In-Al-O is applied to the channel layer. A sample of In-Ga-O applied to the transistor of the channel layer, and a sample of the transistor to which the In-Al-O is applied to the channel layer is produced and evaluated, as applied to the channel as described above. In the case of a layer of transistors. A device having a ratio of various components between indium and gallium (a device having a ratio of various components between indium and aluminum or gallium), and a device exhibiting excellent crystal characteristics are selected. The characteristics of the transistor to be compared are illustrated in Figures 3A and 3B. As illustrated in FIG. 3A, In-B-O exhibits the maximum switching ratio in a combination of B (boron), aluminum, and gallium of indium and a third group element. As is apparent from Fig. 3B, when In-B-O is used, a transistor having the minimum S value can be realized. Therefore, it becomes apparent that the amorphous oxide semiconductor uses a dimetal element as a channel material of the TFT, and the In-B-O system is excellent.

Example 3

In this example, a TFT device is produced in which a non-crystalline In-Zn-B-O oxide semiconductor system is applied to a channel layer. This TFT has the structure illustrated in Fig. 8B.

A polyethylene terephthalate (PET) film was prepared as a substrate. The channel length and channel width of the transistor are 60 micrometers and 180 micrometers, respectively.

First, on the PET substrate 10, the gate electrode 15 is formed by lithography and a photoresist peeling method, and the gate insulating layer 12 is formed and patterned.

The gate electrode 15 is made of a molybdenum film having a thickness of 50 nm. The gate insulating layer 12 is deposited by sputtering to a SiO _x film having a thickness of 150 nm. The specific dielectric constant of the SiO _x film is about 3.7.

Secondly, the channel layer of the transistor is formed by sputtering and patterning through lithography and a photoresist stripping method. The channel layer is made of an amorphous oxide based on In-Zn-B-O, which contains indium, zinc and boron at a composition ratio of In:Zn:B=4:6:1.

An amorphous oxide film based on In-Zn-B-O is formed by radio frequency sputtering in a mixed atmosphere of argon and oxygen.

In this example, three targets (sources of material) are used to form a film by simultaneous deposition. The three targets are sintered materials of two sizes of In ₂ O ₃ , B ₂ O ₃ and ZnO, respectively. An oxide film having a desired In:Zn:B composition ratio is obtained by separately controlling the input RF power for those targets. The atmosphere is set such that the total pressure is 0.5 bar and the gas flow ratio is Ar:O ₂ =100:1. The substrate temperature was set to 25 degrees Celsius.

The thus formed oxide film was found to be an amorphous film because no significant diffraction peak was detected in the X-ray diffraction (film method, incident angle: 0.5 degree). The thickness of the amorphous oxide film is about 30 nm.

An optical absorption spectroscopy revealed that the formed amorphous oxide film had a forbidden band energy width of about 3 electron volts and was transparent with respect to visible light.

The source electrode, the drain electrode, and the gate electrode are made of a transparent conductive film including In ₂ O ₃ :Sn, and each electrode has a thickness of 100 nm.

The thus fabricated TFT was evaluated from the viewpoint of characteristics.

The switching ratio of this exemplary transistor formed on a PET film measured and evaluated at room temperature exceeded 10 ⁹ . The calculated field effect mobility is approximately 5 cm ² /Vs.

Furthermore, the thin film transistor using In-Zn-B-O for the channel according to this example has high performance and high environmental stability.

When the element ratio B/(In+Zn+B) of the amorphous oxide material is 0.05 or higher and 0.29 or lower, excellent transistor operation is ensured.

Compared with a thin film transistor using In-Zn-O which does not contain boron as the channel, the thin film electrocrystal system using this In-Zn-BO based oxide semiconductor as the example of the channel is also Improvement in the stability of the environment (feature fluctuations during storage in the atmosphere are small).

While the invention has been described with reference to the preferred embodiments, the invention The scope of the following patent application is to be accorded the broadest description

10. . . Substrate

11. . . Channel layer

12. . . Gate insulation

13. . . Source electrode

14. . . Bipolar electrode

15. . . Gate electrode

twenty one. . . Substrate

twenty two. . . Insulation

twenty three. . . Source electrode

twenty four. . . Bipolar electrode

25. . . Channel layer

31. . . sample

32. . . aims

33. . . Vacuum pump

34. . . Vacuum gauge

35. . . Substrate fixture

36. . . Gas flow controller

37. . . pressure controller

38. . . Film deposition chamber

111. . . Base

112. . . Channel layer

113. . . Source electrode

114. . . Bipolar electrode

115. . . Gate insulation

116. . . Gate electrode

117. . . Interlayer insulation

118. . . electrode

119. . . Luminous layer

120. . . electrode

121. . . film

122. . . film

123. . . Cell

201. . . Transistor

202. . . Transistor

203. . . Capacitor

204. . . Electroluminescent layer

205. . . Electrode line

206. . . Electrode line

207. . . Electrode line

Figure 1 is a graph illustrating the transfer characteristics of a field effect transistor based on indium-boron-oxygen.

Figure 2 is a graph illustrating the temporal variation in the resistivity of a field effect transistor based on indium-boron-oxygen.

3A and 3B are diagrams illustrating a field effect transistor based on indium-boron-oxygen, a field effect transistor based on indium-aluminum-oxygen, and a field effect electric based on indium-gallium-oxygen. Comparison of features among the crystals, wherein Figure 3A illustrates the switching ratio to be compared, and Figure 3B illustrates the S values to be compared.

Figure 4 is a graph illustrating the transfer characteristics of a field effect transistor based on indium-boron-oxygen having various element ratios B/(In+B).

Fig. 5 is a graph showing the element ratio B/(In+B) dependence of the field effect mobility.

Fig. 6 is a graph illustrating the element ratio B/(In+B) dependence of the threshold voltage.

Fig. 7 is a graph showing the element ratio B/(In+B) dependence of the S value.

8A, 8B, and 8C are cross-sectional views illustrating an example of the structure of a field effect transistor in accordance with an embodiment of the present invention.

9A and 9B are graphs illustrating an example of the characteristics of a field effect transistor in accordance with an embodiment of the present invention.

Figure 10 is a schematic view showing the structure of a thin film forming apparatus for fabricating the field effect transistor.

Figure 11 is a graph showing the element ratio B / (In + B) dependence of the resistivity of a semiconductor film based on indium-boron-oxygen.

Figure 12 is a schematic cross-sectional view showing an example of a display in accordance with the present invention.

Figure 13 is a schematic cross-sectional view showing another example of a display in accordance with the present invention.

Figure 14 is a schematic structural view illustrating a display in which pixels are two-dimensionally arranged, each pixel including an organic EL (electroluminescence) device and a thin film transistor.

Figure 15 is a graph showing the temporal change in the resistivity of the indium-boron-oxygen based semiconductor film.

Claims (10)

A field effect transistor comprising at least one channel layer, a gate insulating layer, a source electrode, a drain electrode, and a gate electrode formed on a substrate, wherein the channel layer comprises An oxide material of at least indium and boron, and wherein the oxide material has an element ratio B/(In+B) of 0.1 or higher and 0.29 or lower. The field effect transistor of claim 1, wherein the element ratio B/(In+B) of the oxide material is 0.1 or higher and 0.22 or lower. The field effect transistor of claim 2, wherein the element ratio B/(In+B) of the oxide material is 0.12 or higher and 0.2 or lower. The field effect transistor of claim 1, wherein the gate insulating layer is made of yttrium oxide. The field effect transistor of claim 4, wherein the channel layer and the gate insulating layer are deposited by sputtering. The field effect transistor of any one of claims 1 to 5, wherein the field effect transistor system is incorporated into a display comprising a pixel device, and wherein the electrode system of the pixel device and the field effect One of the source electrode and the drain electrode of the crystal is connected. The field effect transistor of claim 6, wherein the pixel device is an electroluminescent device. The field effect transistor of claim 6, wherein the pixel device is a liquid crystal cell. The field effect transistor of claim 6, wherein the pixel device is one of a plurality of pixel devices and one of the field effect transistor system and the plurality of field effect transistors, and wherein the plurality of pixel devices and The complex field effect transistor system is two-dimensionally disposed on a substrate. The field effect transistor of claim 1, wherein the oxide material is a non-crystalline oxide material.